Temporal window of integration estimated by omission in bone-conducted ultrasound.

Bone-conducted ultrasound (BCU) can be heard for both normal-hearing and some profoundly deaf individuals. Moreover, amplitude-modulated BCU can transmit the speech signal. These characteristics of BCU provide the possibility of the developing a bone-conducted ultrasonic hearing aid. Previous studies on the perception mechanism of speech-modulated BCU have pointed to the importance of temporal rather than frequency information. In order to elucidate the perception of speech-modulated BCU, further investigation is need concerning the processing of temporal information. The temporal processing of air-conducted audible sounds (ACASs) involves the integration of closely presented sounds into a single information unit. The long-temporal window of integration was estimated approximately 150-200 ms, which contribute to the discrimination of speech sound. The present study investigated the long-temporal integration system for BCU evaluated by stimulus omission using magnetoencephalography. Eight participants with normal hearing took part in this study. Ultrasonic tone burst with the duration of 50 ms and frequency of 30 kHz was used as the standard stimulus and presented with steady onset-to-onset times or stimulus-onset asynchronies (SOAs). In each sequence, the duration of the SOAs were set to 100, 125, 150, 175, 200, or 350 ms. For deviant, tones were randomly omitted from the stimulus train. Definite mismatch fields were elicited by sound omission in the stimulus train with an SOA of 100-150 ms, but weren't with an SOA of 200 and 350 ms for all participants. We found that stimulus train for BCUs can be integrated within a temporal window of integration with an SOA of 100-150 ms, but are regarded as a separate event when the SOA is 200 or 350 ms in duration. Therefore, we demonstrated that the long-temporal window of integration for BCUs estimated by omission was 150-200 ms, which was similar to that for ACAS (Yabe et al. NeuroReport 8 (1997) 1971-1974 and Psychophysiology. 35 (1998) 615-619). These findings contribute to the elucidation and improvement of the perception of speech-modulated BCU.

Autocorrelation factors and intelligibility of Japanese monosyllables in individuals with sensorineural hearing loss.

Some Japanese monosyllables contain consonants that are not easily discernible for individuals with sensorineural hearing loss. However, the acoustic features that make these monosyllables difficult to discern have not been clearly identified. Here, this study used the autocorrelation function (ACF), which can capture temporal features of signals, to clarify the factors influencing speech intelligibility. For each monosyllable, five factors extracted from the ACF [Φ(0): total energy; τ1 and ϕ1: delay time and amplitude of the maximum peak; τe: effective duration; Wϕ(0): spectral centroid], voice onset time, speech intelligibility index, and loudness level were compared with the percentage of correctly perceived articulations (144 ears) obtained by 50 Japanese vowel and consonant-vowel monosyllables produced by one female speaker. Results showed that median effective duration [(τe)med] was strongly correlated with the percentage of correctly perceived articulations of the consonants (r = 0.87, p < 0.01). (τe)med values were computed by running ACFs with the time lag at which the magnitude of the logarithmic-ACF envelope had decayed to -10 dB. Effective duration is a measure of temporal pattern persistence, i.e., the duration over which the waveform maintains a stable pattern. The authors postulate that low recognition ability is related to degraded perception of temporal fluctuation patterns.

Perception of speech in cartilage conduction.

OBJECTIVE: By attaching a transducer to the aural cartilage a relatively loud sound is audible even with a negligibly small fixation pressure applied to the transducer. This form of conduction is referred to as cartilage conduction (CC). Utilizing CC, novel audio devices can be developed, and one possible application is a CC hearing aid. However, there are no studies on speech perception in CC. In this study, CC speech recognition performance was measured and compared with that for air and bone conduction (AC and BC, respectively). METHODS: Nine volunteers with normal hearing participated in the study. The performance-intensity functions were measured for AC, BC and CC. These measurements were performed in the conditions with and without an earplug. RESULTS: Without the earplug, no differences in speech recognition scores were observed among AC, BC, and CC. With the earplug, the level at which the maximum speech recognition score was obtained did not increase in CC, which agreed with the result of BC but not AC. The maximum speech recognition CC score decreased with the earplug. The performance-intensity functions for AC and BC shifted in parallel with the earplug. These shifts approximated the average threshold shifts. In contrast, for CC, the performance-intensity function did not shift in parallel with the earplug. As for the CC threshold shifts with the earplug, although the threshold at 500Hz decreased by 15.4dB, those at 2000 and 4000Hz increased by 13.8 and 31.1dB, respectively. Compared with AC and BC, CC excessively emphasized low over high frequency sounds when the earplug was inserted. Confusion matrices analysis demonstrated that 4%, 22%, and 74% of the errors occurred at low, intermediate, and high frequency speech sounds, respectively. Thus, this excessive low frequency sound emphasis probably prevented the recognition of high frequency speech sounds. CONCLUSION: The decrease in the maximum speech recognition score for CC with the earplug was derived from the biased frequency composition. It can be improved by frequency composition adjustment.

Long-term effects of electrotactile sensory substitution therapy on balance disorders.

This clinical research investigated whether a new type of rehabilitation therapy involving the use of a vestibular substitution tongue device (VSTD) is effective for severe balance disorders caused by unilateral vestibular loss. Sixteen patients with postural imbalances because of unilateral vestibular loss underwent training with VSTD. The VSTD transmits information on the head position to the brain through the tongue as substitutes for the lost vestibular information. The device's electrode array was placed on the tongue and participants were trained to maintain a centered body position by ensuring the electrical signals in the center of their tongue. All participants completed 10 min training sessions 2-3 times per day for 8 weeks. Functional gait assessments and the dizziness handicap inventory were, respectively, used to the evaluate participants' dynamic gait function and their severity of balance problems before and after the training period. All examined parameters improved after the 8-week training period. These changes were maintained for up to 2 years after the termination of the training program. Short-term training with VSTD had beneficial carry-over effects. VSTD training might represent a useful rehabilitation therapy in individuals with persistent balance disorders and might lead to long-term improvements in their balance performance and ability to perform daily and social activities.

An examination of the effects of broadband air-conduction masker on the speech intelligibility of speech-modulated bone-conduction ultrasound.

Ultrasound can be heard by bone-conduction, and speech-modulated bone-conducted ultrasound (BCU) delivers the speech information to the human ear. One of the recognition mechanisms is the demodulation of the signals. Because some of the profoundly deaf can also hear speech-modulated BCU, another mechanism may also contribution to the recognition of speech-modulated BCU. In this study, eight volunteers with normal hearing participated. The intelligibilities of speech-modulated BCU were measured using a numeral word list under masking conditions. Because the masker can mask the demodulated sounds, the evaluation of the masking reveals the contribution of the demodulation to the recognition of speech-modulated BCU. In the current results, the masking of speech-modulated BCU differed from that of original non-modulated speech. Although the masking shifted the recognition curve for the original speech upward, the same results were not observed for the speech-modulated BCU. The masking generated the difference in the correct answers among the words for the speech-modulated BCU. The current results suggested the importance of the envelope of the modulated ultrasonic signal to the recognition under masking condition. Both demodulation and direct ultrasonic stimulation contribute to the recognition of speech-modulated BCU for the normal hearing individuals, and the direct ultrasonic stimulation plays an important role in the recognition for the profoundly deaf.

Cartilage conduction hearing.

Sound information is known to travel to the cochlea via either air or bone conduction. However, a vibration signal, delivered to the aural cartilage via a transducer, can also produce a clearly audible sound. This type of conduction has been termed "cartilage conduction." The aural cartilage forms the outer ear and is distributed around the exterior half of the external auditory canal. In cartilage conduction, the cartilage and transducer play the roles of a diaphragm and voice coil of a loudspeaker, respectively. There is a large gap between the impedances of cartilage and skull bone, such that cartilage vibrations are not easily transmitted through bone. Thus, these methods of conduction are distinct. In this study, force was used to apply a transducer to aural cartilage, and it was found that the sound in the auditory canal was amplified, especially for frequencies below 2 kHz. This effect was most pronounced at an application force of 1 N, which is low enough to ensure comfort in the design of hearing aids. The possibility of using force adjustments to vary amplification may also have applications for cell phone design.

Residual inhibition of tinnitus induced by 30-kHz bone-conducted ultrasound.

Sounds at frequencies of >24-kHz are classified as ultrasound which cannot be heard by humans if presented by air conduction, but can be perceived if presented by bone conduction. Some research studies involving ultrasonic hearing have reported that tinnitus is masked by bone-conducted ultrasound (BCU). However, little is known about residual inhibition (RI), which is a continuous reduction or disappearance of tinnitus after presentation of BCU. This study investigated whether RI could be induced by BCU. Five types of the masker sounds were used to measure RI in 21 subjects with tinnitus. A bone-conducted 30-kHz pure tone was used as a BCU, and an air-conducted 4-kHz pure tone, narrow-band noise, white noise, and a bone-conducted 4-kHz pure tone were used as controls of audible sounds. The masker intensities of the 30-kHz BCU and audible sounds were set at the minimum masking levels of tinnitus plus 3 and 10 dB, respectively, considering the narrow dynamic range of BCU. The duration of RI induced by the 30-kHz BCU was significantly longer than those induced by the 4-kHz sounds, but was not significantly different from that induced by the white noise. BCU activates the cochlear basal turn in response to the high frequency, which may broadly overlap with the frequency range that included the dominant tinnitus pitch in most of our subjects. The longer RI duration for the 30-kHz BCU was probably derived from this characteristic. These results suggested that the peripheral stimulation characteristic of BCU probably contributed to inducing long RI durations.

Evaluation of prosodic and segmental change in speech-modulated bone-conducted ultrasound by mismatch fields.

Speech-modulated bone-conducted ultrasound (BCU) can transmit speech sounds for some profoundly deaf individuals. Hearing aids using BCU are considered to be a novel hearing system for such individuals. In our previous study, the speech discrimination for speech-modulated BCU was objectively confirmed using a magnetoencephalography. Moreover, in our previous behavioral study, prosodic information for speech-modulated BCU could also be discriminated in the normal hearing. However, the prosodic discrimination for speech-modulated BCU has not objectively been studied. In order to evaluate the prosodic discrimination for speech-modulated BCU, mismatch fields (MMFs) elicited by prosodic and segmental change were measured for speech-modulated BCU and air-conducted speech. Ten Japanese participants with normal hearing took part in this study. Stimuli re-synthesized from the speech of a native Japanese female adult were used. Standard stimulus was /itta/ with a flat pitch pattern, and two deviant stimuli were /itta?/ with a rising pitch pattern and /itte/ with a flat pitch pattern. All and nine participants elicited the prominent MMF elicited by the prosodic and segmental change for the speech-modulated BCU, respectively. The moment of MMF components for speech-modulated BCU was significantly smaller than those for air-conducted speech, while no difference in the MMF latency elicited by the prosodic and segmental change were observed between both stimulus conditions. Comparing the MMFs elicited by prosodic and segmental change, no significant differences were observed for both stimulus conditions. Thus, it is suggested that the prosodic change can be discriminate to the same degree as segmental change even for speech-modulated BCU. However, discrimination capability for speech-modulated BCU is slightly inferior to that for air-conducted speech.

Human ultrasonic hearing is induced by a direct ultrasonic stimulation of the cochlea.

Ultrasound can be perceived by bone-conduction. The cochlear basal turn is involved in processing bone-conducted ultrasound (BCU) information. Previous studies have suggested that ultrasonic perception is induced by ultrasound itself. In contrast, it has also been suggested that a lower frequency sound is generated in non-linear process during the transmission pathway to the cochlea to induce an auditory sensations. To address this issue, we assessed cisplatin-induced changes in BCU sensitivity at 27, 30 and 33kHz in 20 participants (40 ears) who were scheduled to undergo cisplatin chemoradiation therapy. Following the treatment, 62.5% ears were diagnosed with hearing loss according to the criteria of the American Speech-Language-Hearing Association. As expected, significant increases in sensitivity threshold were observed for air-conducted sounds ranging from 8 to 14kHz. In contrast, the BCU threshold significantly decreased after the treatment. Considering that both air-conducted high-frequency sound and BCU are perceived in the cochlear basal turn, these findings indicate that ultrasonic perception is independent of hearing a lower frequency sound generated in non-linear process. In addition, our findings support the hypothesis that ultrasound itself induces ultrasonic perception in the cochlea. The observed cisplatin-induced increase in BCU sensitivity may be explained by hypersensitivity associated with outer hair cells' disorder.

Peripheral perception mechanism of ultrasonic hearing.

Ultrasound can be perceived by bone conduction, and its characteristics differ from those of air-conducted audible sound (ACAS) in some respects. Despite many studies on ultrasonic hearing, the details have not yet been clarified. In this study, to elucidate the perception mechanism, the masking of bone-conducted ultrasound (BCU) produced by ACAS and the sensitivity of BCU in hearing impaired subjects were evaluated. We found that BCU was masked by high frequency ACAS, especially in the frequency range of 10-14 kHz. The most effective masker frequency depended on masker intensity. For hearing impaired subjects, the pure tone thresholds at 1-8 kHz and the maximum audible frequencies at cut-off intensities of 70-100 dB HL were significantly associated with the BCU threshold (p < 0.01 or p < 0.05). No subjects with estimated total loss of the inner hair cell system in the cochlear basal turn could hear BCU. These results suggest the peripheral perceptual region to be located in the cochlea. The results of masking show the faster excitation spread to the lower frequency range, depending on the intensity. This faster excitation spread may be due to nonlinearity in cochlear mechanics, which may work even without cochlear amplifier, and induce unique characteristics of BCU.

N1m amplitude growth function for bone-conducted ultrasound.

CONCLUSION: N1m growth indicates the differences in central auditory processing between bone-conducted ultrasound and air-conducted audible sound. OBJECTIVES: Bone conduction enables ultrasound to be heard by the human ear. Despite many studies, the perceptual mechanism of bone-conducted ultrasound has not yet been clarified completely. Therefore, this study investigated the ultrasonic perception of humans, especially as regards the effects of stimulus intensity or loudness. SUBJECTS AND METHODS: The effect of the stimulus level on N1m amplitude was measured over the psycho-acoustical dynamic range. RESULTS: The dynamic range for 30 kHz bone-conducted ultrasound (18.2 +/- 3.3 dB) was found to be significantly narrower than that for 1 kHz air-conducted sound (85.9 +/- 11.9 dB). As the stimulus level increased, the N1m amplitude in response to bone-conducted ultrasound grew faster than that to air-conducted sound. Although the growth of the N1m amplitude for air-conducted sound saturated below the uncomfortable loudness level (UCL), that for bone-conducted ultrasound continued to grow above the UCL.

Comparison between bone-conducted ultrasound and audible sound in speech recognition.

CONCLUSION: This study showed that it is possible to transmit language information using bone-conducted ultrasound (BCU) in normal-hearing subjects. Our results suggest the possibility of a difference in speech recognition between BCU and air-conducted audible sound (ACAS). OBJECTIVE: Ultrasound was audible when delivered by bone conduction. Some profoundly deaf subjects as well as normal-hearing subjects can discriminate BCU whose amplitude is modulated by different speech sounds. These findings suggest the usefulness of developing a bone-conducted ultrasonic hearing aid (BCUHA). However, the characteristics of BCU are still poorly understood. The aim of the present study was to compare BCU and ACAS in terms of their associated speech perception tendency and to investigate the different perceptual characteristics of BCU and ACAS. SUBJECTS AND METHODS: Speech discrimination tests using both BCU and ACAS were performed with normal-hearing subjects. BCU and ACAS were compared for intelligibility and hearing confusion. RESULTS: With BCU, the maximum percentage correct totaled about 75%. Our comparison of the hearing confusion with ACAS and BCU according to the individual syllabic nuclear group showed a clear difference in the incorrect rates. In addition, the stimulus nuclear groups were often perceived in other nuclear groups in BCU.

Assessment of ability to discriminate frequency of bone-conducted ultrasound by mismatch fields.

According to previous studies, ultrasound can be perceived through bone conduction and ultrasound amplitude modulated by different speech sounds can be discriminated by some profoundly deaf subjects as well as the normal-hearing. These findings suggest the usefulness of development of a bone-conducted ultrasonic hearing aid (BCUHA) for profoundly deaf subjects. In this study, with a view to developing a frequency modulation system in a BCUHA, the capability to discriminate the frequency of sinusoidal bone-conducted ultrasound (BCU) was evaluated by measuring mismatch fields (MMF). We compared MMFs between BCU (standard stimuli were 30 kHz, and deviant stimuli were 27 and 33 kHz) and air-conducted audible sound (ACAS; standard stimuli were 1 kHz, and deviant stimuli were 900 and 1100 Hz). MMFs were observed in all subjects for ACAS, however, not observed in a few subjects for BCU. Further, the mean peak amplitudes of MMF for BCU were significantly less than those for ACAS. These findings indicate that the discrimination capability of frequency of sinusoidal BCU is inferior to that of ACAS. It was also demonstrated that normal hearing could to some extent discriminate differences in frequency in sinusoidal BCU. The results indicate a possibility of transmission system for language information making use of frequency discrimination.

Effect of masker frequency on N1m amplitude in forward masking.

The effect of frequency on N1m has been investigated by various methods. However, it has not yet been measured using forward masking. In this study, the frequency specificity of N1m was investigated using forward masking. Although the masker frequency had some influence on N1m amplitudes, the results suggested that the frequency specificity of N1m was worse than that of a single-neuron or psychological tuning curve. This is probably because N1m includes various components, both frequency-specific and non-specific, some of which may be less affected by masking. Thus, our results agree with those of previous studies using intervening tones that suggested widespread neural representation in the auditory cortex.

Effect of a forward masker on the N1m amplitude: varying the signal delay.

Auditory sensation is affected by a forward masker, and this phenomenon has been demonstrated in a neural adaptation model and a temporal window (integration) model. To study forward masking in the central auditory system, the growth of the N1m amplitude was measured by varying the signal delay. In the adaptation model, the masking increases as the signal delay decreases. However, in our results, the minimum N1m amplitude was observed at a signal delay of 40 ms. As the signal delay decreased from 40 ms, the N1m amplitude increased although the masking increased. Our results suggest that the growth of the N1m amplitude largely depends on temporal integration at signal delays below 40 ms.

Effect of stimulus duration for bone-conducted ultrasound on N1m in man.

Ultrasound can be heard by bone conduction in man. However, there has been no consensus about the perception mechanism of bone-conducted ultrasound (BCU). In the current study, to clarify the central auditory system of BCU, the effects of stimulus duration for 30 kHz BCU on N1m were compared with those for air-conducted 1 kHz tone bursts by magnetoencephalography. As a result, the growth of N1m amplitude for both stimuli saturated at the duration of 40 ms, which suggest that the temporal integration system of BCU is similar to that of audible sound. However, significant differences in the growth were observed below the saturation points. The results indicate a possibility that there are some differences in the central auditory system between BCU and audible sound.